…and that is the screenshot from my mobile.
Somehow, the Show Live button on the Chrome browser (on my Chromebook) does not force a refresh.
Has this something to do with the fact that I am running a Python script (server.py)?

I see a node index.js process running. How do I make that “refresh” again automatically?

Yes, Glitch is built around Node.js primarily although other things like Python do work. In this case, it looks like the changes aren’t reflected until your server restarts and changing the .css file doesn’t do that automatically. If you don’t need Python specifically, I’d recommend you use Node.js on Glitch.